Light emitting diodes(LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, an LED produces light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized or part of the light from a single LED must be converted to light of a second wavelength, which is mixed with the light from the original LED. For example, an LED-based white light source that provides an emission that is perceived as white by a human observer can be constructed by combining light from arrays of red, blue, and green emitting LEDs that are generating the correct intensity of light at each color. Similarly, light of other spectral emissions can be produced from the same arrays by varying the intensity of the red, blue, and green LED outputs to produce the desired color output. The intensity of light from each array can be varied by varying the magnitude of the current through the LED or by switching the LEDs on and off with a duty cycle that determines the average intensity of light generated by the LEDs.
A light source designer typically knows the desired output color for a light source in terms of standardized red, blue, and green light intensities. In principle, a light source constructed from red, blue, and green LEDs can be utilized provided the intensities of the light from the individual colors is adjusted to match the required red, blue, and green intensities. Unfortunately, the LED fabrication process provides LEDs having emissions and efficiencies that vary somewhat from one LED to another. If the designer constructs an LED lighting system by assuming that the LEDs are all the same, the variations lead to color shifts in the perceived spectrum of the light. Such variations are often unacceptable. One solution to this problem involves selecting the LEDs such that the selected LEDs have precisely the correct emission efficiency and spectrum. Unfortunately, this solution reduces the production yield and cost increases.
In principle, each light source can be adjusted to provide the desired output spectrum. Such a process involves determining the current to be applied to each of the colored arrays of LEDs in each light source by varying the currents and examining the light source output with a standardized camera. An LED light source system with spectral feedback (“LED lighting feedback system”) can be constructed using the above described principle. A standardized camera continually sends measurement information to the light source controller, which adjusts the driving current to the LEDs. A standardized camera may be one that is configured to respond closely to the CIE color matching function (CMF). Such a camera will produce measurements that correspond to the CIE standard color scheme. Cameras that correspond to other standards may also be used. These standardized cameras are usually expensive because their responses are tuned to correspond to the standard spectral responses. The CIE color matching function is an example of a standard spectral response. A less expensive alternative is to utilize a CMOS tri-color sensor that is sensitive to the red, green and blue region of the visible spectrum. These sensors are commercially available and have constructions that are similar to CMOS cameras used in PDAs and mobile phones. These sensors typically do not conform to a standard color scheme. One problem with using such sensors is that a calibration procedure is required to map the spectral responses of the sensor to the LED light source spectral output. This requires the manufacturer of the LED lighting feedback system to install and maintain this type of calibration equipment on the manufacturer's production line as well as setting the calibration values for each light source produced. This increases the capital investment needed to establish the production line. If the manufacturer of the LED lighting feedback system is supplied with compound light sources that emit light of known CIE coordinates, then the calibration procedure, although still necessary, becomes less expensive and simpler because the calibration values for each compound light source is known without measurement.